The present invention claims priority under 35 U.S.C. .sctn..sctn. 119 and/or 365 to Hei 10-154951 filed in Japan on Jun. 3, 1998, the entire content of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a control signal processor for processing control signals as in an excitation system of a generator and a power system stabilizer (hereinafter sometimes referred to as "PSS") using the control signal processor.
2. Description of the Prior Art
In the generator excitation system, a deviation (from a reference value) of a terminal voltage of the generator has heretofore been used as a control signal of an automatic voltage regulator (hereinafter sometimes referred to as "AVR") which holds the generator voltage constant. Further, a deviation of the active power of the generator, a deviation of the rotational speed of the generator, and a frequency deviation of the generator voltage are commonly used as PSS control signals for suppressing variations in the quality or state of the voltage and active power of the generator due to an accident in the power system. These deviation signals may sometimes vary from their steady-state values owing to measurement noise and mechanical vibrations and changes of operating conditions. Such noise and varying components are detrimental components of the AVR or PSS control signal, and unless removed, they may cause a malfunction of the automatic voltage regulator AVR or power system stabilizer PSS.
Since the frequency of power system oscillation modes are generally within a certain range (about 0.1 to 3 Hz), the conventional control signal processor uses only an input signal having a frequency within the specified frequency range, as an effective control signal, and attenuates the other frequency components, thereby removing the abovementioned detrimental components.
To attenuate such harmful components, it is general practice in the art to employ, as a control signal processor, a high-frequency filter which has a transfer function of 1/(TS+1) (where T is a constant and S a Laplace variable) as shown in FIG. 7, or a low-frequency filter which has a transfer function of TS/(TS+1) as shown in FIG. 8. These filters are implemented by a variety of circuits from simple analog circuits to complex digital circuits. The basic feature of these filters is to attenuate some of frequency components of all input signals or to separate the frequency components according to modes and pass them or attenuate them for each mode. The higher the accuracy of the filter, the more complex its circuit configuration and the slower its response.
FIG. 9 illustrates in block form a conventional power system stabilizer. Reference numeral 90 denotes an input signal indicating the rotational speed of the generator; 91 denotes a high-frequency filter which attenuates the high-frequency component, such as shown in FIG. 7; 92 denotes a phase compensator which compensates for all phases of the high-frequency filter 91 of the preceding stage, a low-frequency filter 93 of the following stage and the automatic voltage regulator AVR into which the output signal of the power system stabilizer PSS is input; 93 denotes the low-frequency filter which attenuates the low-frequency component, such as depicted in FIG. 8; 94 denotes an amplifier for amplifying the level of the output signal from the low-frequency filter; 95 denotes a limiter for limiting the output signal level of the amplifier 94 to a predetermined range of voltage level; and 96 denotes the PSS output signal, which is input into the automatic voltage regulator AVR not shown.
Next, the operation of the prior art example will be described.
The input signal 90 is processed to remove its high-frequency noise component and varying component, and its phase delay with the high-frequency filter 91, the low-frequency filter 93, and the phase comparator 92. The output signal from the phase compensator 92 is fed to the low-frequency filter 93, wherein its low-frequency noise component and varying component are removed by the low-frequency filter 93. The filter output is amplified by the amplifier 94 and converted by the limiter 95 to a signal within a predetermined range of voltage levels, which is provided therefrom as the output signal 96.
In the conventional power system stabilizer PSS, it is necessary that a phase delay of the automatic voltage regulator AVR, other than the phase delay by the high-frequency filter 91 and the low-frequency filter 93 be compensated by the phase compensator 92. Further, to maintain good control efficiency in the frequency region in which power varies, the output level also needs to have suitable values. With the conventional configuration, however, it is difficult to satisfy all the requirements, no matter how precisely the PSS parameters are controlled.
The conventional control signal processor and the power system stabilizer using the processor are constructed as described above, and hence they have such problems as listed below.
1. When noise superimposed on the control signal and its varying high- or low-frequency components are attenuated by the high- and low-frequency filters, the phase of the signal within the effective signal range will shift; since it is difficult in many cases to fulfil the aim of control while compensating for the phase shift, much difficulty is encountered in constructing the optimum control system. A filter of the type that does not cause a phase shift may be available, but its application is impractical because of complex configuration and low response speed or expensive.
2. Since all input signals are applied to filters, a wide frequency range of the effective control signal allows easy incorporation thereinto of unwanted harmful components, whereas a narrow frequency range incurs attenuation of the high- or low-frequency component contained in the effective control signal, too, lessening the effect of control.
3. Noise by torsional vibrations of the generator shaft is a harmful component, but since its frequency is close to the local oscillation frequency of the generator, its attenuation is also accompanied by the attenuation of the local oscillation component that is the effective control signal.
The solution of these problems requires a control signal processor which eliminates harmful components and, at the same time, does not deform the effective control signal but is simple in circuit configuration and fast in response.